Ammoniation of straw using urea, ammonia gas or ammonium hydroxide

Deir El-Hajjar Research Station, Ministry of Agriculture and Agrarian Reform, Syria

(Present addresses: *Agricultural Research Institute, Nicosia, Cyprus; **FAO, Animal Production and Health Division, Via delle Terme di Caracalla, 00100 Rome, Italy; ***Pasture, Forage and Livestock Program, International Center for Agricultural Research in the Dry Areas, PO Box 5466, Aleppo, Syria)

Chopped cereal straw (3 sacks/treatment) was treated in polyvinyl chloride (PVC) sacks (1.5 m diameter, 2.8 m height and 0.5 to 0.8 mm thickness) with urea-solution (10% urea solution at the rate of 400 litres/100 kg straw), NH₄OH (12 litres of 25% NH₃ by weight/100 kg straw) or NH₃-gas (3 litres/100 kg straw). In another test, the effect of straw moistening (none, 8 or 16 litres of water/100 kg) on the efficiency of NH₃ gas in upgrading straw was studied. All chemicals significantly increased the in vitro organic matter digestibility (control 40.2%, NH₃ gas 55.2%, NH₄OH 53.9% and urea 51.5%) and content of N x 6.25 (control 37, NH₃-gas 109, NH₄OH 104 and urea 122 g/kg DM) of straw. Moistening of straw did not affect the digestibility (no water 55.2%, 8 litres water 55%, 16 litres water 54.8%) or CP content (no water 107, 8 l water 112, 16 l water 108 g CP/Kg DM). For all three chemicals there was no difference in straw digestibility whether it was taken from the top, middle or bottom of the sack and/or at the beginning, middle or end of the radius.

KEY WORDS: Barley straw, ammoniation, urea, ammonia gas, ammonium hydroxide, digestibility

Cereal straws are produced in considerable quantities in many countries representing the most widely underutilized source of carbohydrates. The limited use of cereal straws is due to their low voluntary intake and low digestibility, associated with high lignin content, the manner in which this indigestible material is bound to the digestible cellulose and hemicellulose, and the low nitrogen concentration (Coxworth et al 1977).

Chemical treatment of crop residues with various alkalis has increased digestibility and often has increased voluntary intake and animal performance (Jackson 1978). Use of ammonia gas, ammonium hydroxide and urea has attracted considerable attention in recent years (Sundstol and Coxworth 1984) as a means of adding nitrogen and increasing digestibility and voluntary intake simultaneously.

The present paper reports on the effect of ammonia gas, NH₄OH and urea in upgrading the nutritional value of straw.

Polyvinyl chloride (PVC) sacks (1.5 m diameter, 2.8 m height and 0.5 to 0.8 mm thickness) were used for storing/treating chopped straw. Three sacks (replicates) were used per treatment. Sacks were filled with straw and sealed prior to injection of NH₃ gas and or sealed after the injection with water, NH₄OH and urea solution. Injection of water or NH₄OH was made in three heights using a perforated metal pipe and a hand operated pump.

The treatments were: (US) untreated straw, (NH₃) straw injected with ammonia gas, (8NH₃) straw moistened with 8 litres of water/100 kg straw and injected with ammonia gas, (16NH₃) straw moistened with 16 litres of water/100 kg straw and injected with ammonia gas, (NH₄OH) straw injected with NH₄OH and (Urea) straw sprayed with urea solution.

Water and NH₄OH were injected through a perforated metal pipe placed in the centre of the sack through its top (mouth). Pumping of water/solution was done at three different heights. The sack was sealed following the spraying of urea solution (10% urea solution at the rate of 400 ml/kg straw) and/or injection of NH₄OH. Twelve litres of 25% NH₃ by weight, 3 litres ammonia gas and 40 litres of 10% urea solution were used per 100 kg straw.

NH₃ gas was injected through a perforated metal pipe which was pressed through the PVC and into the middle of the sack. The hole left in the PVC sack after the withdrawal of the pipe was sealed with tape. The ammonia bottle was upside down on a rack placed on a platform balance and the ammonia was added by weight.

The PVC sacks were opened 42 days after treatment and samples of straw were taken at three heights (top, middle, bottom of the sack) and at each height at three points of the radius (centre, middle, end of radius). Samples at all heights were taken following 24 h aeration.

Samples of straw were exposed to air for 24 h before being dried (55 C), ground (1 mm) and analysed for total N (AOAC 1975). Digestibility in vitro was determined on the samples following the method of Tilley and Terry (1963). The data were analysed by one- way analysis of variance (Steel and Torrie 1960).

There was no mould development in any of the treatments. When water was injected at different points in the centre of the sack the colour of straw was variable being darker for the moistened straw treated with NH₃ gas. Similarly, straw injected with NH₄OH was darker close to the centre of the sack. There was no difference in the ammonia smell of straw treated with NH₄OH or ammonia gas. Contrary, ammonia smell was weaker in the urea-treated straw.

The point of sampling for a particular chemical did not affect the in vitro digestibility of treated straw (Table 1). There were however, significant differences in the N x 6.25 content which was higher (P<0.05) on the top compared to middle and bottom for ammonia gas and urea treatment. In contrast, NH₄OH treated straw obtained from the bottom of the sack had a significantly (P<0.05) higher N x 6.25 content than that from the top point.

Table 1. Digestible organic matter and N x 6.25 content of straw obtained at different parts of the sack (Top, middle and bottom)

	Top	Middle	Bottom	SE

NH3
N x 6.25 (g/kg DM)	115a	103b	104b	1.8
DOM (% of DM)	55.2	54.6	54.6	0.38

NH4OH
N x 6.25 (g/kg DM)	89b	104ab	118a	8.1
DOM (% of DM)	49.8	54.3	57.5	2.22

Urea
N x 6.25 (g/kg DM)	200a	97b	92b	17.7
DOM (% of DM)	56.7	49.9	49.3	2.55

All chemicals were effective in increasing (P<0.001) the N x 6.25 content and the in vitro digestibility of straw (Table 2). There were no significant differences, however, between chemicals, for in vitro digestibility, and nitrogen retained in the straw.

The similarity of results with aqueous and anhydrous NH₃ is in line with the data of Sundstol et al (1978) and Orskov et al (1983). Furthermore, the higher proportion of N retained with urea than NH₃ gas is in line with the data of MacDearmid et al (1988) but at variance with those of Joy et al (1992).

Addition of water to dry straw did not affect the effectiveness of ammonia gas in increasing the digestibility and the nitrogen content of treated straw (Table 3). This is at variance with the findings of Wais et al (1972) and Sundstol et al (1979) where addition of water before treatment enhanced the ammoniation process. Harton and Steacy (1979), however, underlined the impracticability of adding water in the straw prior to treatment. Furthermore, in the studies of Mandell et al (1988), addition of water to raise the moisture content of wheat straw up to 25% did not result in significant improvements over straw ammoniated at 15% moisture. Similarly, chemical treatment at 30% moisture resulted in limited improvements in the digestion of DM and OM.

Table 2. The effect of chemical on the nutritive value of cereal straw

	No. of observations	N x 6.25 (g/kg DM)	DOM (% of DM)

Treatment
Untreated	3	37b	40.2b
NH₃-gas	9	109a	55.2a
NH₄OH	9	104a	53.9a
Urea	9	122a	51.5a
SD		33.6	3.86

Table 3. The effect of added water on the efficiency of ammonia (NH3) gas in upgrading cereal straw

	No of observations	N x 6.25 (g/kg DM)	DOM (%)

NH₃-gas	9	107	55.2

NH₃-gas + water
8 litres	9	112	55.0
16 litres	9	108	54.8
SE		2.6NS	0.97NS

It is concluded that all three chemicals (urea solution, anhydrous or aqueous ammonia) are equally effective in upgrading the nutritional value of straw and that selection of the chemical will depend upon the available infrastructure which in the case of urea and aqueous ammonia is cheap and simple.

The authors are grateful to the FAO and UNDP offices in Damascus, to Drs S Badawi (Chief AGON,FAO, Rome), R Sancousy (Feed Resources, AGAP, FAO, Rome) and K Qamar (AGON, FAO, Rome) for continuous support and encouragement and to his Excellency the Minister of Agriculture and Agrarian Reform and his staff for making available all necessary facilities for the implementation of this work. This work was carried out in conjunction with the Greater and Improved Use of Agricultural Residues for Animal Feeding Project, a joint undertaking between the Government of Syria and the UNDP and FAO.

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